The following type of pipe coupling is generally frequently used to connect or disconnect a path for supplying a high-pressure fluid. The pipe coupling incorporates a valve element opened or closed by a manual operation. The operation for opening or closing the valve element is conducted as follows. The valve element is opened after the completion of connection between a socket and a plug that constitute the pipe coupling. The plug is disconnected from the socket after the completion of the operation for closing the valve element. Thus, the operating procedure is restricted.
A conventional pipe coupling of the above-described type is disclosed in Japanese Patent No. 2,694,302. This pipe coupling has three valve elements constituting an inlet valve member, a vent valve member, and an outlet valve member, respectively. The three valve elements are disposed in series in the pipe coupling. Accordingly, the overall length of the pipe coupling is unfavorably long, and the pressure loss is undesirably large. Consequently, the packing efficiency is degraded.
Under these circumstances, a pipe coupling has been proposed in which a ball valve is used as a valve element for opening or closing a fluid passage, thereby reducing the pressure loss while making the whole structure compact.
However, if a conventionally known ball valve is employed as it is, in a pipe coupling for a high-pressure fluid under a high-pressure environment, the following problem arises. In the pipe coupling, a seal is effected between the ball valve and the primary fluid passage portion of a socket fluid passage. Therefore, the pressure of a fluid acting in the primary fluid passage portion of the socket fluid passage is applied as a large back pressure to a cylindrical seal member that is disposed in the primary fluid passage portion in such a manner as to be in pressure contact with the ball valve to seal between the ball valve and the inner wall of the socket fluid passage.
Accordingly, the cylindrical seal member offers a large resistance to the ball valve as it is rotated, making it difficult to perform the operation for opening or closing the ball valve. Further, because a high pressure acts only on the primary fluid passage side of the ball valve, a ball rotating shaft which rotates the ball valve is undesirably decentered by the pressure. This also makes it difficult to perform the ball valve open-close operation.
Further, the cylindrical seal member is strongly pressed against the ball valve under the high pressure of the fluid. Therefore, the seal member may be worn away and damaged by the opening edge of the valve bore of the ball valve when rotated.